Have you ever said, "Speak of the devil"? Have you ever appeared when a conversation was going on and you heard someone say, "Speak of the devil"? If so, you might have figured out they were talking about you and you showed up. It was not that they were calling you "the devil." To say, "Speak of the devil" goes back to hundreds of years. The phrase is old and appears in various Latin and Old English texts from the 16th century.
"Speak of the devil" is a phrase used to acknowledge the coincidence of someone appearing at the exact time that his or her name is called or being talked about. This does not mean that the person is mean, evil, or is the devil. Instead, it is a joking way of acknowledging the person's appearance at that exact moment that the person's name is mentioned.
"Speak of the devil" is the short form of the idiom "Speak of the devil and he doth appear" or its alternative form "speak of the devil and he shall appear." The phrase is used when an object of discussion unexpectedly becomes present during the conversation. The person just mentioned appears based on the superstition that if you mention the devil's name, he will appear.
"Speak of the devil' is an expression derived from the Middle Ages was a superstitious prohibition against speaking directly of the devil or of evil in general, which was considered to incite that it would appear, generally with unfortunate consequences.
After reading this article, the next time you hear the expression you will have a better idea what "speak of the devil" really means.